[css-fonts-4] first available font, see w3c#4796

Author: svgeesus

css-fonts-4/Overview.bs

@@ -3681,11 +3681,13 @@ given an identical set of available fonts and rendering technology.
 The <dfn export>first available font</dfn>,
 used for example in the definition of <a>font-relative lengths</a> such as ''ex''
 or in the definition of the 'line-height' property,
-is defined to be the first available font
-that would match the U+0020 (space) character
-given font families in the 'font-family!!property' list
+is defined to be the first font
+whose 'unicode-range' includes the U+0020 (space) character,
+given the font families in the 'font-family!!property' list
 (or a user agent's default font if none are available).
 
+Note: it does not matter whether that font actually has a glyph for the space character.
+
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